P4.5 Electromagnetic Effects

A conductor moving across a magnetic field, or a changing magnetic field linking a conductor, induces an e.m.f..

• Faster movement → larger e.m.f.
• Stronger magnetic field → larger e.m.f.
• More turns → larger e.m.f.

A rotating coil cuts magnetic field lines, inducing an alternating e.m.f.

• Slip rings maintain contact.
• Output is an alternating voltage.

A current in a wire produces a magnetic field.

• Straight wire → circular field lines.
• Solenoid → strong, uniform field.
• More current → stronger field.

A conductor carrying current in a magnetic field experiences a force.

• Reverse current → force reverses.
• Reverse field → force reverses.
• Use Fleming’s Left-Hand Rule.

A current-carrying coil in a magnetic field experiences a turning effect.

• More turns → stronger turning effect.
• More current or stronger field → stronger turning effect.
• Split-ring commutator keeps rotation in one direction.

A transformer uses a soft-iron core and two coils to change voltage.

• Primary: input coil.
• Secondary: output coil.
• Step-up: \( N_s > N_p \).
• Step-down: \( N_s < N_p \).

Key Equations

Voltage ratio: \( \dfrac{V_p}{V_s} = \dfrac{N_p}{N_s} \)

Efficiency: \( I_p V_p = I_s V_s \)

Power loss: \( P = I^2 R \)